A variety of diseases affect the viability and function of motor neurons located within the spinal cord and projecting to muscle throughout the body. In order to develop an effective gene therapy application to treat MN disease it will be necessary to efficiently transduce MNs in vivo. Herpes simplex virus type 1 (HSV-1) has been engineered to be a versatile gene vector for transduction of sensory nerves in vivo, the natural site for wild-type virus latency. While high multiplicity infection of spinal cord will result in virus transduction of motor neurons (MNs), replication defective HSV vectors fail to transduce motor neurons in adult rodent models following skin or muscle inoculation. A similar result is likely to occur in human gene therapy applications since HSV is not found in MNs in natural infections. Thus treatment of MN diseases will require the enhancement of HSV vector infection of MNs with the longer-range goal of limiting the virus host-range to MN receptors. Because little is known about the effects of introducing new receptor binding ligands into the HSV-1 envelope, the applicants will explore strategies to enhance and eventually retarget HSV infection to MNs through modification of the receptor binding activities of three fundamentally different envelope components, glycoproteins B, C, and D. Because tetanus toxin receptors (TTR) are specifically found on MN synaptic junctions, we will focus our efforts on using nontoxic TTR binding components of the tetanus toxin engineered to be presented on the virus envelope as recombinant glycoproteins capable of initiating virus attachment and entry into MNs both in vitro and in vivo. Aim 1 is designed to target HSV to MNs by substitution of the native heparan sulfate attachment determinants presented by HSV glycoproteins B and C (gB and gC) with a tetanus toxin (TTx) heavy chain C-terminal fragment (HcC). Aim 2 is intended to restrict virus infection via HveB binding and evaluate re-targeting of gD via HcC. Aim 3 is designed to test soluble HveC in combination with HcC ligand bearing gC and gB or HveC-HcC soluble fusion proteins as effectors of MN targeting. The soluble HveC adapter will be expected to block gD binding to cellular HveC while directly or indirectly re-targeting the virus to TTx receptors.